The present invention is generally directed to a burner and more specifically, a two-stage burner with a diffusion flame upstream of a catalytic reactor.
Fuel cells are electrochemical devices that convert a fuel""s energy directly to electrical energy. A Proton Exchange Membrane (PEM) fuel cell produces electricity by first separating hydrogen into hydrogen ions and electrons with the aid of a catalyst within an anode loop. The hydrogen ions then pass through an electrolyte membrane from the anode loop to a cathode side of the fuel cell. The electrons, which can not penetrate the membrane, flow through an external circuit in the form of electric current. On the cathode side of the fuel cell, oxygen from the air combines with the hydrogen ions to form water. When the fuel cell power plant is at rest, the anode loop seeks equilibrium with the cathode side; thus oxygen (a constituent of air present of the cathode side) passes through the membrane and builds up in the anode loop.
During startup of the fuel cell, the anode loop, which is a closed loop, has relatively pure hydrogen gas injected into it from a storage source. As the anode loop employs a catalyst in the form of a platinum coating, to help separate the hydrogen gas into ions and electrons, high concentrations of hydrogen in conjunction with oxygen in the presence of a catalyst could be an explosive combination. Therefore, oxygen levels within the anode loop must be controlled to permit the introduction of hydrogen. Similarly at shutdown, hydrogen levels must be controlled to stop the flow of current and to again eliminate the potential explosion hazard of a hydrogen and oxygen mixture, or the release of hydrogen into the ambient air.
Controlling the levels of oxygen, or hydrogen, within the anode loop has been accomplished by purging the loop with an inert gas. Purging has been used prior to system startup as well as after system shutdown. Purging, however, requires a storage system for an inert gas, such as nitrogen. In a mobile fuel cell this type of purging is not a desirable option, as nitrogen would have to be transported with the ancillary requirement for recharging. In addition, purging for startup of a mobile fuel cell, such as the power source in a car, must be accomplished within acceptable timeframes to consumers, which generally do not wish to wait more than a couple of seconds to start their cars.
It is therefore the objective of the present invention to provide an alternative method and structure for controlling the levels of oxygen on startup and the levels of hydrogen at shutdown within the anode loop. It is a further objective of this invention to accomplish the degree of control required within a timeframe acceptable to consumers.
The invention is directed in one aspect to a structure and method that employs oxidation to deplete to an acceptable level an undesired constituent from a closed loop system. In the specific application of a PEM fuel cell, the invention can be employed to deplete oxygen at startup and hydrogen at shutdown from the anode loop.
The invention comprises a housing having an entrance and exit. Downstream of the housing entrance is an injector. Downstream of the injector is a flame stabilizer that creates a recirculation zone for anchoring a diffusion flame. Further downstream of the flame stabilizer is an igniter. The igniter is positioned such that it is within or proximate to the recirculation zone created by the flame stabilizer.
Downstream of the igniter is a catalytic module having an upstream surface. The catalytic module is placed sufficiently downstream of the igniter such that it is downstream of the diffusion flame. The catalytic module can be made using any well known technique for making a catalyst bed, including but not limited to catalyst deposited on or alloyed with such supports as screen, expanded metal, foam, gauze or ceramic monolith, or catalyst in the form of pellets. In the preferred embodiment, short channel screens with catalyst deposited thereon were used. The catalyst is selected based upon the reaction contemplated. Common catalysts for hydrogen and oxygen reactions are platinum and palladium.
In a refinement of the invention, a first heat exchanger is positioned between the igniter and the catalytic module. The heat exchanger can be passive or active. It is desired that the first heat exchanger be positioned sufficiently downstream of the igniter such that it is beyond the diffusion flame. A second heat exchanger, passive or active, can be positioned downstream of the catalytic module.
To assure timely catalytic module light off, sufficient catalytic activity, a heating element can be placed proximate to the upstream surface of the catalytic module. In the preferred embodiment an electrical resistive heating element was woven into the upstream surface of the catalytic module. A separate heating element is also acceptable. The heating element need not be electric and the present invention should not be considered so limited.
The method of operation of the present invention relies on a two-step oxidation process to deplete the undesired constituent, fuel or oxidant. The first oxidation step relies on diffusion burning of a stratified mixture. Within the stratified mixture at the boundaries between the fuel and oxidant, pockets of fuel and oxidant within flammability limits will exist. The igniter initially ignites these pockets, with additional pockets being ignited therefrom, and the pockets will continue to react by diffusion burning until the undesired constituent, fuel or oxidant, is consumed to such a point that the pocket is no longer within the flammability limit. The resulting reactive products and remaining fuel and oxidant mixture then pass into the downstream catalytic module where catalytic oxidation completes the reaction of the undesired constituent.
It is a matter of design choice as to the degree of depletion of the undesired constituent the present invention accomplishes. While diffusion burning within a pocket can only deplete the undesired constituent to the lower flammability limit, catalytic combustion can essentially deplete the undesired constituent to for all practical purposes zero. The level of ultimate depletion is design specific.
The use of the present invention for startup of a fuel cell is as follows. At startup the undesired constituent within the anode loop is oxygen and it must be depleted to a level such that hydrogen can be safely added to the anode side to start the PEM reactor. First, a feed stream in the anode loop comprising oxygen is enhanced with an additional fuel that can be oxidized in the presence of the oxygen. In the case of a mobile fuel cell, hydrogen is desired as the additional fuel as it is already onboard; however, other fuels could be used. The amount of additional fuel that is added to the feed stream is at least equal to the amount needed to react the desired amount of oxygen to reach the desired depleted oxygen condition. A target depletion level for the oxygen is one-half of one percent by volume. Additional fuel beyond that required for the desired depletion level could be added but not so much that the pockets will exceed the upper flammability limit, or the concentration is high enough after exiting the burner to initiate a reaction within the cell stack.
After the additional fuel is added to the feed stream if the stoichiometry, the ratio of fuel to oxygen, is within the flammability limits, fuel will be oxidized by diffusion burning. The flame will self extinguish when the oxygen has been consumed such that the stoichiometry is below the lower flammability limit.
The balance of the oxygen and fuel then enters the catalytic module and continues to be oxidized in the presence of the catalyst. Unlike diffusion burning which requires that the stoichiometry be within flammability limits, catalytic oxidation has no such limitation. Thus catalytic oxidation further reduces the amount of oxygen in the feed stream down to potentially almost zero. The method of the present invention, therefore, has the advantage over one-step diffusion burning in that oxygen levels can be reduced below the concentration level fixed by the lower flammability limit.
The method of the present invention also has advantages over one-step catalytic oxidation. To accomplish desired levels of depletion within a single pass through a catalytic module, the catalytic module would have to operate at near stoichiometric conditions producing high temperatures. Material limits of the catalytic module, substrate and/or catalyst, limit the ability to operate at this condition. To accomplish desired levels of depletion therefore, multiple passes through the catalytic module would be required. Multiple passes through the catalytic module means the feed gas would circulate around the anode loop multiple times thereby heating the membrane, unless a heat exchanger to moderate burner exit gas temperatures that would slowly rise with each pass was provided.
In one specific application, it has been estimated that a one-step catalytic system would require 10 passes of the fuel and oxidant mixture through the catalytic module (10 loops around the anode loop), requiring 20 seconds. The combined system of the present invention would accomplish the desired depletion within three loops, or about three seconds. Times will vary depending on system layouts and plumbing volumes.
The use of the present invention for shutdown of the fuel cell is as follows. For shutdown, the anode loop contains pure hydrogen, oxygen, generally as a constituent of air, is added at the feed stream to oxidize the hydrogen. The amount of oxygen added is at least equal to the amount needed to oxidize the desired amount of hydrogen to reach the desired depleted hydrogen condition, for example below a few hundred parts per million (ppm) by volume. Excess oxygen, however, is preferred to assure complete oxidation of the hydrogen. Again depleted does not necessarily mean all hydrogen has been removed, only that the hydrogen has been removed to the level desired, which is application dependent.
After the additional oxygen is added to the hydrogen feed stream, if the stoichiometry, the ratio of hydrogen to oxygen, is within the flammability limits, hydrogen fuel will be oxidized by diffusion flame burning. The flame will self extinguish when the hydrogen has been consumed such that the stoichiometry is below the lower flammability limit, around 4% hydrogen by volume. In order to remove additional hydrogen, the remaining hydrogen continues to be oxidized in the presence of the downstream catalyst.